1. Field of the Invention
The present invention relates to a fuel cell stack formed by stacking a plurality of electrolyte electrode assemblies each including an anode, a cathode, and an electrolyte between the anode and the cathode. Each of the electrolyte electrode assemblies is interposed between separators.
2. Description of the Related Art
In recent years, various types of fuel cells are developed. For example, a solid polymer electrolyte fuel cell is known. The solid polymer electrolyte fuel cell employs a membrane electrode assembly (MEA) which comprises two electrodes (anode and cathode) and an electrolyte membrane interposed between the electrodes. The electrolyte membrane is a polymer ion exchange membrane (proton exchange membrane). Each of the electrodes comprises a catalyst and a porous carbon sheet. The membrane electrode assembly is interposed between separators (bipolar plates). The membrane electrode assembly and the separators make up a unit of the fuel cell (unit cell) for generating electricity. A plurality of unit cells are connected together to form a fuel cell stack.
In the fuel cell of the fuel cell stack, a fuel gas such as a hydrogen-containing gas is supplied to the anode. The catalyst of the anode induces a chemical reaction of the fuel gas to split the hydrogen molecule into hydrogen ions (protons) and electrons. The hydrogen ions move toward the cathode through the electrolyte, and the electrons flow through an external circuit to the cathode, creating a DC electric current. An oxygen-containing gas or air is supplied to the cathode. At the cathode, the hydrogen ions from the anode combine with the electrons and oxygen to produce water.
Generally, an oxygen-containing gas supply passage and a fuel gas supply passage (reactant gas supply passages) extend through the fuel cell stack in the stacking direction for supplying an oxygen-containing gas and a fuel gas (reactant gases) to the cathode and the anode, respectively. Further, an oxygen-containing gas discharge passage and a fuel gas discharge passage extend through the fuel cell stack in the stacking direction for discharging the fuel gas and the oxygen-containing gas and the fuel gas from the cathode and the cathode.
The water produced in the chemical reactions on the power generation surfaces of the electrodes is likely to flow into the oxygen-containing gas discharge passage, and the water is retained in the oxygen-containing gas discharge passage. Further, the water may also be condensed, and retained in the fuel gas discharge passage. The retained water undesirably narrows or closes the oxygen-containing gas discharge passage and the fuel gas discharge passage to prevent the flows of the oxygen-containing gas and the fuel gas. As a result, power generation performance is decreased.
For example, Japanese laid-open patent publication No. 2001-236975 discloses a fuel cell stack which was made in an attempt to solve the problem. The fuel cell stack has an oxygen-containing gas supply passage and an oxygen-containing gas discharge passage in communication with the oxygen-containing gas supply passage. The oxygen-containing gas supply passage and the oxygen-containing discharge passage extend in the stacking direction. Additionally, a bypass plate having a bypass passage is provided remotely from an outlet (discharge port) of the oxygen-containing gas discharge passage. An inlet of the bypass passage is connected to the oxygen-containing gas supply passage and an outlet of the bypass passage is connected to the oxygen-containing gas discharge passage. The oxygen introduced into the oxygen-containing gas supply passage partially flows into the inlet of the bypass passage, flows out of the outlet of the bypass passage, and flows into the oxygen-containing gas discharge passage. Thus, the water retained remotely from the outlet of the oxygen-containing gas discharge passage is desirably pushed out by the flow of the oxygen-containing gas. The retained (condensed) water generated in the chemical reactions is smoothly discharged from the fuel cell stack. Therefore, the power generation performance is not decreased.
At the time of starting the operation of the fuel cell stack, or at the time of restarting the operation of the fuel cell stack after a temporary interruption of the operation, condensed water may be present in the pipes for supplying the reactant gases (oxygen-containing gas and/or fuel gas) to the body of the fuel cell stack.
In particular, in the pipe for supplying the oxygen-containing gas, a large amount of condensed water may be generated. At the time of starting the operation of the fuel cell stack, the condensed water may drip onto the power generation surfaces (reaction surfaces) of the electrodes near a reaction gas inlet (supply port). Due to the condensed water, the supply of the reaction gases may not be carried out smoothly. Therefore, the power generation performance of unit cells may be decreased.
In the fuel cell stack, a plurality of unit cells are electrically connected. Each of the unit cells outputs an electric current of an identical level. Therefore, if any of the unit cells outputs an electric current of a low level, all of the unit cells output the low electric current. Thus, the power generation performance of the overall fuel cell stack is limited by the low electric current. If the unit cells are operated continuously with the low power outputting capability due to the condensed water, the unit cells may be damaged by the electric current beyond the capability of the unit cells.
For example, Japanese laid-open patent publication No. 2000-6718 discloses a fuel cell stack which was made in an attempt to solve the problem. In the fuel cell stack, a dehumidifying mechanism is disposed between the end plates at opposite ends of the fuel cell stack in the stacking direction. The dehumidifying mechanism is positioned in the gas passage which extends from a reactant gas supply port to the first unit cell. The dehumidifying mechanism has a chamber for adjusting the amount of water in the reactant gas depending on the temperature of the unit cell, and discharging means for discharging the excessive water from the chamber.
When the reactant gas flows into the chamber from the reactant gas supply port, the amount of water in the chamber is adjusted depending on the temperature of the unit cell adjacent to the chamber by discharging the excessive water from the chamber using the discharging means. In this manner, the humidity in the reactant gas to be supplied to the unit cell is suitably maintained, and water condensation is effectively prevented.